Electromigration is an electrochemical process where metal on an insulating
material, in a humid environment and under an applied electric field, leaves
its initial location in ionic form and redeposits somewhere else. Such migration
may reduce isolation gaps and ultimately lead to an electrical short circuit.
The process begins when a thin continuous film of water has been formed and
a potential is applied between oppositely charged electrodes.
Positive metal ions are formed at the positively biased electrode1
(the anode), and migrate toward the negatively charged cathode. Over time, these
ions accumulate as metallic dendrites, reducing the spacing between the electrodes,
and eventually creating a metal bridge.
Whilst most often seen as a surface effect, vertical migration can also occur
when moisture has penetrated into the bulk of a porous material.
Dendritic growth across tracks on a PCB
Dendrite growth across resist
The rate of electromigration increases with temperature and has four prerequisites
– a mobile metal; a voltage gradient; a continuous film of moisture; soluble
- Silver is the metal most susceptible to migration, since it is anodically
very soluble and requires a low activation energy to initiate the migration
process. Copper, zinc, and lead will also migrate, although only under much
more severe conditions. Most other common electronic materials are not susceptible
to migration: iron, nickel, and tin because of their low solubility in water;
gold, platinum, and palladium because they are anodically stable.
- The severity of electromigration increases with applied potential gradient,
the time to grow dendrites decreasing both with reducing electrode spacing
and increasing voltage. The time to failure is a log-linear function of voltage
at low voltages, but reaches a minimum as the effect becomes transport limited
by the number of ions available.
- How much moisture is needed has been hotly debated. Whether the moisture
needed represents just few monolayers, or several hundred monolayers, depends
on the nature, structure, and porosity of the surface, and on its affinity
for water. For example, phenolic resin laminates are more hygroscopic than
ceramic substrates and are thus more susceptible to migration.
- The type of surface has another effect on electromigration, because nucleation
sites are required for dendrites to initiate and grow, and these are provided
by the surface roughness of the substrate and the presence of sharp corners
- The rate of electromigration increases with relative humidity, but will
become significantly worse if changing conditions encourage the formation
of water droplets. This has been reported as a main factor in MLC failure
due to tin and silver migration.
- A soluble ionic species is essential to provide the conductive medium for
the migration to occur, and the nature of the ionic contaminants has a major
impact on performance. The severity of electromigration depends on the particular
ionic species involved, and factors such as its mobility, ionic radius, electronegativity,
electron affinity, and charge to size. Ionic contaminants come from a wide
range of sources such as activators in the flux, reaction products of the
soldering process, the breakdown of cleaning solvents, and fingerprints.
The move to finer pitch components, and consequently reduced spacings, makes
electromigration more likely, because the voltage gradients between conductors
increase and it becomes more difficult to eliminate the minute amounts of ionic
contaminants which are sufficient to cause problems.
Three main methods have been used to reduce the electromigration problem:
- Alloying any silver with an anodically stable metal such as palladium or
platinum. This is one reason (as well as the improvement in solderability)
why palladium-containing silver alloys are preferred to pure silver for MLC
- Using a coating of solder resist to shield the PCB surface from humidity
and ionic contaminants. Exposed epoxy glass is much more hydrophilic than
most solder mask materials, and the improvement in Surface Insulation Resistance
is typically several orders of magnitude
- Plating any silver with metals such as tin, nickel, or gold. In MLCs, the
nickel barrier coating improves resistance to electromigration as well as
enhancing performance on exposure to molten solder.
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Author: Martin Tarr
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